By way of example, U.S. Pat. No. 4,546,760 issued to Suzuki et al discloses an artificial heart driving apparatus which alternately supplies an air of a given positive pressure and a suction of a given negative pressure. Air discharged from an air compressor is fed through a first positive pressure open/close valve to a positive pressure accumulator, the pressure of which is detected by a pressure sensor, with the first open/close valve being controlled to be turned on and off so that the accumulator maintains a pressure within a given range. Air of a positive pressure from the positive pressure accumulator is supplied to an artificial heart through a second positive pressure open/close valve by opening the latter during the systole of the artificial heart. On the other hand, a negative pressure from a decompressor (vacuum suction unit) is applied through a first negative pressure open/close valve to a negative pressure accumulator, the pressure of which is also detected by a pressure sensor, with the first open/close valve being controlled to be turned on and off, so that the negative pressure accumulator maintains a pressure within a given range. The negative pressure from the negative pressure accumulator is applied to the artificial heart through a second negative pressure open/close (O/C) valve by opening the latter during the diastole of the artificial heart.
The positive and the negative pressure alternately applied to the artificial heart are substantially equal to the pressures which prevail in the positive and the negative pressure accumulator, respectively. In this manner, the first positive pressure and negative pressure valves in combination with the positive pressure and the negative pressure sensor are effective to determine the magnitude of the positive pressure and the negative pressure which are effective to drive the artificial heart for contraction and expansion, respectively, while the second positive pressure and negative pressure valves are effective to switch between the positive and the negative pressure as fed to the artificial heart.
It will be appreciated that when either accumulator has an increased capacity, the magnitude of a reduction in the internal pressure of that accumulator when the second valve is changed from its closed to its open condition remains low, and a variation in the magnitude of a pressure supplied to the artificial heart which is caused by the opening and closing of the first valves also remains small. If the capacity of the accumulator is reduced or the provision of such accumulator is omitted, there occurs an overhunting of an increased magnitude when the pressure supplied to the artificial heart rises to a given value in response to a switching of the second valve from its closed to its open condition, and there also occurs a greater variation in the magnitude of pressure supplied to the artificial heart which occurs in response to the opening and closing of the first valve for pressure regulating purposes. In consideration of these aspects, the cited U.S. Patent chooses a capacity of the accumulator on the order of 4,000 cc, for example, so that the overhunting of the pressure supplied to the artificial heart in response to a change of the second valve from its closed to its open condition as well as a variation in the pressure supplied to the artificial heart in response to the opening/closing of the first valve be maintained small. If the capacity of the accumulator is reduced to the order of 300 cc or less, there occurs a variation in the pressure supplied to the artificial heart during the systole and the diastole which is excessively high.
It will be noted that in the arrangement mentioned above, a positive pressure drive system requires a first and a second open/close valve and a negative pressure drive system also requires a first and a second negative pressure open/close valve, thus requiring an increased number of valve units. The provision of the second valves is essential in order to achieve the pumping operation, and it may be noted that the first valves may be omitted by providing accumulators of sufficient capacity and providing a constant torque control of the compressor and the decompressor or adding a constant pressure relief valve. However, there may be experienced a difficulty in regulating the driving pressure, in particular, in achieving a pressure regulation of a relatively small magnitude.